Computational scanning microscopy with improved resolution

ABSTRACT

A method of accumulating an image of a specimen using a scanning-type microscope, comprising the following steps:
         Directing a beam of radiation from a source through an illuminator so as to irradiate a surface S of the specimen;   Using a detector to detect a flux of radiation emanating from the specimen in response to said irradiation;   Causing said beam to follow a scan path relative to said surface;   For each of a set of sample points in said scan path, recording an output D n  of the detector as a function of a value P n  of a selected measurement parameter P, thus compiling a measurement set M={(D n , P n )}, where n is a member of an integer sequence;   Using computer processing apparatus to automatically deconvolve the measurement set M and spatially resolve it so as to produce reconstructed imagery of the specimen,
 
wherein, considered at a given point p i  within the specimen, the method comprises the following steps:
   In a first probing session, employing a first beam configuration B 1  to irradiate the point p i  with an associated first Point Spread Function F 1 , whereby said beam configuration is different to said measurement parameter;   In at least a second probing session, employing a second beam configuration B 2  to irradiate the point p i  with an associated second Point Spread Function F 2 , whereby:
           F 2  overlaps partially with F 1  in a common overlap zone O i  in which point p i  is located;   F 1  and F 2  have respective non-overlapping zones F 1 ′ and F 2 ′ outside of O i ,   
           Using a Source Separation algorithm in said computer processing apparatus to perform image reconstruction in said overlap zone O i  considered separately from said non-overlapping zones F 1 ′ and F 2 ′.

The invention relates to a method of accumulating an image of a specimenusing a scanning-type microscope, comprising the following steps:

-   -   Directing a beam of radiation from a source through an        illuminator so as to irradiate a surface S of the specimen;    -   Using a detector to detect a flux of radiation emanating from        the specimen in response to said irradiation;    -   Causing said beam to follow a scan path relative to said        surface;    -   For each of a set of sample points in said scan path, recording        an output D_(n) of the detector as a function of a value P_(n)        of a selected measurement parameter P, thus compiling a        measurement set M={(D_(n), P_(n))}, where n is a member of an        integer sequence;    -   Using computer processing apparatus to automatically deconvolve        the measurement set M and spatially resolve it so as to produce        reconstructed imagery of the specimen.

The invention also relates to a scanning-type microscope in which such amethod can be performed.

Charged-particle microscopy is a well-known and increasingly importanttechnique for imaging microscopic objects, particularly in the form ofelectron microscopy. Historically, the basic genus of electronmicroscope has undergone evolution into a number of well-known apparatusspecies, such as the Transmission Electron Microscope (TEM), ScanningElectron Microscope (SEM), and Scanning Transmission Electron Microscope(STEM), and also into various sub-species, such as so-called “dual-beam”tools (e.g. a FIB-SEM), which additionally employ a “machining” FocusedIon Beam (FIB), allowing supportive activities such as ion-beam millingor Ion-Beam-Induced Deposition (IBID), for example. More specifically:

-   -   In a SEM, irradiation of a specimen by a scanning electron beam        precipitates emanation of “auxiliary” radiation from the        specimen, in the form of secondary electrons, backscattered        electrons, X-rays and photoluminescence (infrared, visible        and/or ultraviolet photons), for example; one or more components        of this flux of emanating radiation is/are then detected and        used for image accumulation purposes.    -   In a TEM, the electron beam used to irradiate the specimen is        chosen to be of a high-enough energy to penetrate the specimen        (which, to this end, will generally be thinner than in the case        of a SEM specimen); the flux of transmitted electrons emanating        from the specimen can then be used to create an image. When such        a TEM is operated in scanning mode (thus becoming a STEM), the        image in question will be accumulated during a scanning motion        of the irradiating electron beam.

More information on some of the topics elucidated here can, for example,be gleaned from the following Wikipedia links:

http://en.wikipedia.org/wiki/Electron_microscope

http://en.wikipedia.org/wiki/Scanning_electron_microscope

http://en.wikipedia.org/wiki/Transmission_electron_microscopy

http://en.wikipedia.org/wiki/Scanning_transmission_electron_microscopy

As an alternative to the use of electrons as irradiating beam,charged-particle microscopy can also be performed using other species ofcharged particle. In this respect, the phrase “charged particle” shouldbe broadly interpreted as encompassing electrons, positive ions (e.g. Gaor He ions), negative ions, protons and positrons, for instance. Asregards ion-based microscopy, some further information can, for example,be gleaned from sources such as the following:

-   http://en.wikipedia.org/wiki/Scanning_Helium_Ion_Microscope-   W. H. Escovitz, T. R. Fox and R. Levi-Setti, Scanning Transmission    Ion Microscope with a Field Ion Source, Proc. Nat. Acad. Sci. USA    72(5), pp 1826-1828 (1975).

It should be noted that, in addition to imaging, a charged-particlemicroscope (CPM) may also have other functionalities, such as performingspectroscopy, examining diffractograms, performing (localized) surfacemodification (e.g. milling, etching, deposition), etc.

Apart from using charged particles as irradiating beam, it is alsopossible to perform scanning microscopy using a photon beam. An exampleof such a technique is so-called confocal microscopy, in which scanningirradiation by a point source of photons stimulates localized emanationof fluorescence radiation from the specimen. A detector can be used tocollect (part of) this flux of fluorescence radiation and accumulate animage on the basis thereof. More information on this topic can, forexample, be gleaned from the following Wikipedia link:

-   http://en.wikipedia.org/wiki/Confocal_microscopy

In all cases, a scanning-type microscope will comprise at least thefollowing components:

-   -   A radiation source, such as a Schottky source or ion gun in the        case of a CPM, or a laser or lamp in the case of an optical        microscope.    -   An illuminator, which serves to manipulate a “raw” radiation        beam from the source and perform upon it certain operations such        as focusing, aberration mitigation, cropping (with an aperture),        filtering, etc. It will generally comprise one or more        (charged-particle) lenses, and may comprise other types of        (particle-)optical component also. If desired, the illuminator        can be provided with a deflector system that can be invoked to        cause its output beam to perform a scanning motion across the        specimen being investigated.    -   A specimen holder, on which a specimen under investigation can        be held and positioned (e.g. tilted, rotated). If desired, this        holder can be moved so as to effect the desired scanning motion        of the beam w.r.t. the specimen. In general, such a specimen        holder will be connected to a positioning system such as a        mechanical stage.    -   A detector, which may be unitary or compound/distributed in        nature, and which can take many different forms, depending on        the radiation being detected. Examples include photomultipliers        (including solid-state photomultipliers, SSPMs), photodiodes,        CMOS detectors, CCD detectors, photovoltaic cells, etc., which        may, for example, be used in conjunction with a scintillator        film, for instance.

Methods as set forth in the opening paragraph above have beenextensively developed in recent years by the assignee of the currentapplication (FEI Company, Hillsboro, Oreg., USA). In particular, thefollowing notable publications deserve mention:

-   (i) U.S. Pat. No. 8,232,523/EP 2 383 768 B1, in which P is a    property of the (incoming) radiation beam—such as beam energy, beam    convergence angle or beam focal depth—and spatial resolution    (deconvolution) of M is performed using a statistical Blind Source    Separation (BSS) algorithm.-   (ii) U.S. Pat. No. 8,581,189/EP 2 557 586 B1, in which P is again a    property of the (incoming) radiation beam—such as beam energy, beam    convergence angle or beam focal depth—and deconvolution of M is    performed using a generalized three-dimensional reconstruction    technique, e.g. on the basis of a Bayesian statistical approach.-   (iii) U.S. Pat. No. 8,586,921/EP 2 557 587 A2, in which P is a    property of the (emanating) radiation flux—specifically emission    angle (e.g. of emitted secondary electrons)—and deconvolution of M    is again conducted using a general volumetric reconstruction    algorithm.-   (iv) U.S. Pat. No. 8,704,176/EP 2 648 208 A2, in which P is again a    property of the (emanating) radiation flux—specifically energy of    emitted electrons—and deconvolution of M is once more achieved using    three-dimensional reconstructive mathematics.

In deconvolving M, one can, for example, spatially resolve it into aresult set R={(V_(k), L_(k))}, in which a spatial variable Vdemonstrates a value V_(k) at an associated discrete depth level L_(k)referenced to the surface S, k being a member of an integer sequence,and spatial variable V representing a physical property of the specimenas a function of position in its bulk, e.g. contrast, intensity, densityvariation, atomic weight, staining concentration, electron yield/X-rayyield, etc., all of which are directly or indirectly determined byphysical characteristics of (the material of) the specimen, and on thebasis of which it is possible to construct an entity such as an image,map or spectrum, for example. In this way, one converts an inherentlydegenerate signal from the specimen into a depth-referenced image stack.A general way of solving this deconvolution problem is (for example) to:

-   -   Define a Point Spread Function (PSF) that, for each value of n,        has a kernel value K_(n) representing a behavior of said        (incoming) beam of radiation in a bulk of the specimen as        perceived by the detector for measurement parameter value P_(n).    -   Define an imaging quantity that, for each value of n, has a        value Q_(n) that is a multi-dimensional convolution of K_(n) and        V, such that Q_(n)=K_(n)*V;    -   For each value of n, computationally determine a minimum        divergence

min D(D _(n) ∥K _(n) *V)

between D_(n) and Q_(n), wherein one solves for V while applyingconstraints on the values K_(n). It will be clear to the skilled artisanthat, although such deconvolution may be depth-referenced, it isgenerally not depth-confined. Because a general PSF will have lateralspread, the deconvolution yields full volumetric imagery of thespecimen, but (if desired) allows such imagery to be rendered on alayer-by-layer basis (“computational slicing” in the depth direction).

The above-mentioned techniques have produced a revolution incomputational electron microscopy, allowing detailed SEM tomography tobe performed as never before. Whereas the examples given abovespecifically involve the use of charged particles (such as electrons) inthe irradiating beam, the basic principles involved can also beexploited in scanning-type optical microscopes, in which the irradiatingbeam comprises photons.

Although techniques such as those set forth above produce satisfactoryresults, the current inventors have worked extensively to improve themeven further. The results of this endeavor are the subject of thecurrent application.

It is an object of the invention to provide an improved method of thetype set forth in the opening paragraph above. In particular, it is anobject of the invention that such a method should offer greater imagingresolution than presently provided by methods of this kind.

These and other objects are achieved in a method as set forth in theopening paragraph, characterized in that, considered at a given pointp_(i) within the specimen, the method comprises the following steps:

-   -   In a first probing session, employing a first beam configuration        B₁ to irradiate the point p_(i) with an associated first Point        Spread Function F₁, whereby said beam configuration is different        to said measurement parameter;    -   In at least a second probing session, employing a second beam        configuration B₂ to irradiate the point p_(i) with an associated        second Point Spread Function F₂, whereby:        -   F₂ overlaps partially with F₁ in a common overlap zone O_(i)            in which point p_(i) is located;        -   F₁ and F₂ have respective non-overlapping zones F₁′ and F₂′            outside of O_(i),    -   Using a Source Separation algorithm in said computer processing        apparatus to perform image reconstruction in said overlap zone        O_(i) considered separately from said non-overlapping zones F₁′        and F₂′.

The crux of the current invention can be set forth in terms ofmathematical considerations (see Embodiments below), but it can also beexplained on the basis of a simplified physical elucidation. Basically,the inventive approach is ensuring that the point p_(i) of the specimenlocated in overlap zone O_(i) will be subjected to (at least) twodifferent regions/portions/aspects of the functional form f_(PSF) of aPSF (which serves to describe physical interaction between the beam andthe specimen). In this way, the technique of the current inventioncauses point p_(i) (and the rest of zone O_(i)) to be “probed” by, forexample:

-   -   A “deeper region” of f_(PSF) during the first probing session        and an “shallower region” of f_(PSF) during the second probing        session; or    -   A “left portion” of f_(PSF) during the first probing session,        and a “right portion” of f_(PSF) during the second probing        session,        etc. The inventive deconvolution procedure accordingly exploits        the fact that the overlap zone O_(i) has been probed by these        (at least) two different regions/portions of f_(PSF), in a        somewhat similar manner to the way in which stereo vision allows        an object to be pinpointed more accurately than mono vision, or        range determination from multiple points allows an object to be        more accurately located (triangulated) than range determination        from just one point. One can alternatively grasp the invention        by considering it in one or more of the following manners:    -   Since the overlap zone O_(i) is, by definition, smaller than the        individual PSFs contributing to it, the inventive approach        provides a finer-scale spatial resolution than “raw” prior-art        techniques such as those set forth above. The present invention        may thus be regarded as producing a super-resolution imaging        result.    -   The invention concentrates on common components of the        overlapping PSFs in isolation from uncommon components, thus        cutting out “dead wood” from the spatial resolution procedure.    -   Since overlap zone O_(i) is inspected in multiple probing        sessions, signal-to-noise ratio is inevitably improved.    -   Each probing of O_(i) by a different region/portion of f_(PSF)        can be regarded as a means to establish a further member of a        set of simultaneous equations describing O_(i); the more        simultaneous equations that one obtains, the more defined the        solution space becomes.        Examples of Source Separation (SS) techniques suitable for use        in the current invention include, for instance:    -   Independent Component Analysis (ICA), which is a technique that        allows a multivariate signal to be separated into additive        sub-components;    -   Principal Component Analysis (PCA);    -   Non-negative Matrix Factorization (NNMF),        etc., which may particularly lend themselves to application in        specific situations. For some general information on such        techniques, see, for example, the following references:

-   http://en.wikipedia.org/wiki/Independent_component_analysis

-   The abovementioned patents U.S. Pat. No. 8,232,523/EP 2 383 768 B1.

-   [1] P. Comon and C. Jutten, Handbook of Blind Source Separation:    Independent Component Analysis and Applications, Academic Press    (2010).

-   [2] A. Hyvärinen and E. Oja, Independent Component Analysis:    Algorithms and Applications, Neural Networks, 13(4-5):411-430    (2000).

-   [3] H. Lantëri, M. Roche, C. Aime, “Penalized maximum likelihood    image restoration with positivity constraints: multiplicative    algorithms, Inverse Problems,” vol. 18, pp. 1397-1419 (2002).

-   [4] I. T. Jolliffe, Principal Component Analysis, Series: Springer    Series in Statistics XXIX, 2nd ed., Springer, N.Y. (2002).

In the context of this recitation and the further elucidation below, thefollowing points should be borne in mind:

-   -   (a) The abovementioned first and second probing sessions may, if        desired, be supplemented by further probing sessions at point        p_(i), thus leading to accumulation of a general set F={F₁, F₂,        F₃, . . . , F_(j), . . . } of partially overlapping PSFs.    -   (b) The inventive procedure at a given point p_(i) can be        repeated at a whole train p={pi} of (successive) points along        (below) the scan path. The associated overlap zones O_(i) will        then effectively “merge” into a larger tract O={O_(i)}, e.g. in        the form of a particular (sub-surface) stratum/volume in the        specimen (see FIGS. 1B and 2B, for example). As explained above,        “standard” spatial resolution can then be realized outside of 0,        but the current invention allows augmented spatial resolution to        be achieved within O.    -   (c) In respect of the preceding item, one has, for example, the        freedom to:        -   (I) Deploy a full set F at point p_(i), and then repeat this            procedure at each subsequent point in p (“full F before each            step in p”); or        -   (II) Deploy PSF F_(j) at each point in p_(i), and then            repeat this procedure for each subsequent PSF in F (“full p            before each step in F”).    -   (d) The various probing sessions at point p_(i) may, in        principle, be performed serially or concurrently, whereby:    -   In the former case (serial probing), point p_(i) is first        irradiated by a first beam having beam configuration B₁, and is        thereafter irradiated by a second beam having beam configuration        B₂, etc.    -   In the latter case (concurrent probing), multiple beams are used        to simultaneously irradiate point p_(i), at a range of        (co-existing) mutually different beam configurations.        The skilled artisan will readily understand these points.

It should be explicitly noted that the present invention isfundamentally different in nature to the inventions set forth in patentdocuments (i)-(iv) above. In documents (i)-(iv), the measurementparameter P is varied as a way of obtaining (convoluted) imaginginformation from a series of (successive) depth layers within aspecimen, i.e. adjustment of parameter P is regarded as a tool forprobing deeper and deeper into the specimen. On the other hand, in thepresent invention, variation of parameter P is effectively a tool totailor the size/shape of a “higher-order” (overlapping) PSF portion(O_(i))—at a given position within a specimen—so as to be able toperform super-resolution image reconstruction in an area smaller thanthe footprint of a probing beam at said position. Lateral scanningmotion of the probing beam can be exploited to extend said position intoa layer (which could be surfacial or sub-surface) but, in its mostfundamental form, the invention does not intrinsically yield bulk/volumeimaging throughout different depths of the specimen (though suchdepth-resolved information could ultimately be obtained by using theextended invention on a (stacked) layer-by-layer basis).

In a particular embodiment of the present invention, the followingspecific aspects apply:

-   -   Said surface is defined to extend parallel to an XY-plane of a        Cartesian coordinate system XYZ    -   Said beam configuration is chosen to be a Z-position of a point        of entry of the beam into the specimen;    -   Between said first and second probing sessions, a physical        slicing procedure is used to remove a layer of material of        thickness L from an initial surface S₁, thereby exposing a new        surface S₂;    -   Point Spread Functions F₂ and F₁ are displaced relative to one        another in said Z-direction by an amount L.        The mechanism of this embodiment can be explained as follows        (see FIG. 1A):    -   Consider p_(i) in the specimen to be located at a Z-distance L+z        below S₁, where z is some positive increment.    -   In the first probing session, the incoming radiation beam B₁        will impinge on S₁ and produce a PSF F₁ that extends down into        the specimen from S₁; point p_(i) will then be located at a        Z-distance L+z into this PSF F₁.    -   A physical slicing procedure (such as ion milling, microtome        cutting, etching, etc.) is now used to remove a layer of        thickness L from old surface S₁, thereby exposing a fresh        surface S₂.    -   In the second probing session, the incoming radiation beam B₂        will impinge on S₂ and produce a PSF F₂ that extends down into        the specimen from S₂; point p_(i) will then be located at a        Z-distance z into this PSF F₂. The PSFs F₁ and F₂ demonstrate a        partial overlap zone O_(i). Beams B₁ and B₂ have a common        propagation axis b₁₂, which extends parallel to the Z-direction.    -   The PSFs F₁ and F₂ can have (approximately) the same functional        form f_(PSF) (e.g. an ovaloid, which starts with a narrow neck        at the point of impingement of the beam, widens as one        progresses into the specimen (lateral spread) and then tapers        again with increasing extinction), but the overlap zone O_(i)        containing p_(i) will be subjected to a different Z-region of        each. This fact allows an SS algorithm (such as ICA) to be used        in performing detector signal deconvolution within zone O_(i),        resulting in augmented spatial resolution in this zone (compared        to the prior art).    -   As set forth in item (b) above, this effect need not be limited        to just the zone O_(i) in which point p_(i) is located; instead,        if lateral scanning is performed during the first and second        probing sessions, then zone O_(i) will be just one component of        a merged tract (e.g. stratum/volume) O, located below S₂, for        which the current invention can realize improved spatial        resolution. See, for example, FIG. 1B in this regard.    -   If desired, the procedure set forth above can be repeated in        further iterations, whereby one progressively ventures deeper        and deeper into the specimen, thus creating a stack of        sub-surface tracts/strata O in which improved spatial resolution        can be obtained. Such a scenario is, for example, illustrated in        FIG. 1C.    -   The skilled artisan will understand that, in the current        situation, approach (I) above cannot be employed, since there is        an irreversible, destructive step (layer removal) between        probing sessions; instead, approach (II) may be employed. In        this context, it should be noted that:        -   A whole measurement set ¹M can be obtained for PSF F₁ (at            each point p_(i) in p), by varying a measurement parameter            ¹P and recording the attendant detector output.        -   Similarly, a whole measurement set ²M can be obtained for            PSF F₂ (at each point p_(i) in p), by varying a measurement            parameter ²P and recording the attendant detector output,            whereby ²P and ¹P may be the same or different.        -   Each of these measurement sets ¹M, ²M separately allows a            spatially resolved image of (part of) the specimen to be            generated; however, using the current inventive insights, a            higher-resolution image may be obtained for the            above-mentioned overlap tract O.

It should be noted that prior-art techniques (i)-(iv) above discuss thecombined use of physical slicing and tomography, but that this is onlyto increase the achievable range of such tomography into the specimen;there is no teaching in the prior art vis-à-vis the exploitation ofoverlapping PSFs in the Z-direction, or the application of SS algorithmsto achieve improved spatial resolution in the overlap zone(s)/tract(s)concerned.

In an alternative embodiment of the current invention, the followingspecific aspects apply:

-   -   Said beam configuration is chosen to be an angle of the beam        relative to the surface S (most generally considered in three        dimensions);    -   Between said first and second probing sessions, said angle of        the beam is adjusted.    -   Point Spread Functions F₂ and F₁ are angled relative to one        another.

The mechanism of this embodiment can, for example, be explained asfollows (see FIG. 2A).

-   -   Define a tilt angle T (declination/pitch) w.r.t. the surface S.    -   In the first probing session, the incoming radiation beam B₁        impinges on S at tilt T₁, and produces a PSF F₁ that extends        into the specimen along propagation axis b₁ and that intersects        sub-surface point p_(i). If T₁≠90°, then such        impingement/extension will be oblique (otherwise it will be        normal/perpendicular).    -   In the second probing session, the incoming radiation beam B₂        impinges on S at a different tilt T₂, producing a PSF F₂ that        extends into the specimen along propagation axis b₂ at a        different slope, and again intersects point p_(i).    -   Since T₂≠T₁, point p_(i) will experience different        regions/aspects of PSFs F₁ and F₂ as measured along their        respective propagation axes b₁, b₂. This fact allows an SS        algorithm (such as ICA) to be used in performing detector signal        deconvolution within zone O_(i), resulting in augmented spatial        resolution in this zone (compared to the prior art).    -   Once again, as set forth in item (b) above, this effect need not        be limited to just the zone O_(i) in which point p_(i) is        located; instead, if lateral scanning is performed during the        first and second probing sessions, then zone O_(i) will be just        one component of a merged tract (e.g. stratum/volume) O, located        below S, for which the current invention can realize improved        spatial resolution. See, for example, FIG. 2B in this regard.

It should be noted that, in addition to a tilt angle T—which measuresdeclination/pitch in a vertical plane (containing the Z-axis)—it is alsopossible to define an azimuthal angle A—which measures orbital angle/yawin a horizontal plane (parallel to the XY plane). In the situation shownin FIG. 2A, the beams B₁ and B₂ have different azimuthal angles (A₁ andA₂, respectively)—in particular, the beams B₁ and B₂ oppose each otherdiametrically, with a difference of 180° between their azimuthal angles;however, this does not have to be the case, and the beams B₁ and B₂could just as validly approach point p_(i) with the same azimuthalangle. Moreover, beams B₁ and B₂ could also approach point p_(i) withthe same tilt angle, but with different azimuth angles. These pointsalso receive attention in Embodiment 1 below.

Care should be taken not to confuse the “variable beam angle” embodimentof the previous paragraph with known—and very different—techniques suchas so-called “micro-rotation”. The technique of micro-rotation can beregarded as being an angular version of so-called confocal imaging,which is linear in nature. In confocal imaging, a focal plane islinearly displaced in (incremental) steps through a specimen, whereas,in micro-rotation, a sample is angularly rotated through a focal plane;in both cases, the intention is to “sweep” the focal plane (eitherlinearly or angularly) through an extended volume of the specimen. Incontrast, in the inventive embodiment of the previous paragraph, thepurpose of beam tilt is to create a localized overlap zone (ofadjustable size/shape/location) between different PSFs, for the purposeof defining a confined region in which super-resolution imagereconstruction is to occur.

In a further embodiment of the current invention, the following specificaspects apply:

-   -   Said beam configuration is selected to be a species of particle        in said beam;    -   Point Spread Functions F₂ and F₁ are mutually different as        regards at least one of size and shape.

The mechanism of this embodiment can be explained as follows (see FIG.3A).

-   -   Consider the term “species” as here employed to refer to        characteristics such as electrically charged or uncharged, sign        of electrical charge, relatively heavy or light, relatively        long- or short-wavelength, etc. In this context, particles such        as electrons, protons, relatively light ions (e.g. He ions),        relatively heavy ions (e.g. Ga ions), photons, soft or hard        X-rays, etc., are considered to be different species of        particle.    -   Such different species of particle will generally demonstrate        different interactions with a given specimen, with associated        differences in the shape and/or size of the attendant PSF.    -   In the first probing session, the incoming radiation beam B₁        comprises a first species of particle. When this beam B₁        impinges on S, it produces—for example—a PSF F₁ that extends to        a relatively deep level into the specimen, but with relatively        little lateral spread. This PSF F₁ intersects point p_(i).    -   In the second probing session, the incoming radiation beam B₂        comprises a second species of particle. When this beam B₂        impinges on S, it produces—for example—a PSF F₂ that extends to        a relatively shallow level into the specimen, but with greater        lateral spread than in the case of beam B₁. This PSF F₂ also        intersects point p_(i).    -   Since PSFs F₂ and F₁ differ in size and/or shape, point p_(i)        will experience different regions/portions/aspects of each. This        fact allows an SS algorithm to be used in performing detector        signal deconvolution within zone O_(i), resulting in augmented        spatial resolution in this zone (compared to the prior art).    -   Once again, as set forth in item (b) above, this effect need not        be limited to just the zone O_(i) in which point p_(i) is        located; instead, if lateral scanning is performed during the        first and second probing sessions, then zone O_(i) will be just        one component of a merged tract (e.g. stratum/volume) O, located        below S, for which the current invention can realize improved        spatial resolution. See, for example, FIG. 3B in this regard.

If desired, various combinations/hybrids of the above-mentionedEmbodiments can be employed, all within the scope of the currentinvention. For example:

-   -   After the situation depicted in FIG. 2B has been enacted, a        physical slicing procedure can be used to remove a layer of        material of thickness L from the specimen, thereby exposing a        new surface (refer to FIG. 1A); the situation shown in FIG. 2B        can then be repeated on this newly exposed surface. A similar        statement applies to the situation depicted in FIG. 3B, for        example.    -   The beams of FIGS. 1A and 3A may, if desired, be shot obliquely        into the specimen rather than normally.        etc.

The invention will now be elucidated in more detail on the basis ofexemplary embodiments and the accompanying schematic drawings, in which:

FIG. 1A renders a cross-sectional view of a specimen that is beingimaged according to a particular embodiment of the current invention.FIG. 1B shows the technique of FIG. 1A applied at multiple laterallydisplaced points. FIG. 1C shows the technique of FIG. 1A iterated atadditional depths.

FIG. 2A renders a cross-sectional view of a specimen that is beingimaged according to another embodiment of the current invention. FIG. 2Bshows the technique of FIG. 2A applied at multiple laterally displacedpoints.

FIG. 3A renders a cross-sectional view of a specimen that is beingimaged according to yet another embodiment of the current invention.FIG. 3B shows the technique of FIG. 3A applied at multiple laterallydisplaced points

FIG. 4 renders a cross-sectional view of an embodiment of ascanning-type microscope according to the present invention.

FIG. 5A depicts an experimental set-up pertaining to an exemplaryembodiment of the current invention. FIG. 5B shows imagery acquired byconsidering only beam B₁ of FIG. 5A and FIG. 5C shows imagery acquiredusing the exemplary embodiment of the current invention.

FIG. 6A shows a conventional FIB-SEM image of the specimen, using aprior-art imaging technique. FIG. 6B depicts imagery pertaining toanother exemplary embodiment of the current invention.

In the Figures, where pertinent, corresponding parts are indicated usingcorresponding reference symbols.

EMBODIMENT 1

FIG. 2A renders a cross-sectional view of a specimen that is beingimaged according to given embodiment of the current invention. Asalready alluded to above, this Figure illustrates the following:

-   -   A first incoming radiation beam B₁ that impinges on specimen        surface S at tilt T₁, and produces a PSF F₁ that extends into        the specimen along propagation axis b₁, and that intersects        sub-surface point p_(i).    -   A second incoming radiation beam B₂ that impinges on S at a        different tilt T₂, producing a PSF F₂ that extends into the        specimen along propagation axis b₂ at a different slope, and        again intersects point p_(i).    -   Since T₂≠T₁, point p_(i) experiences different regions/aspects        of PSFs F₁ and F₂ as measured along their respective propagation        axes b₁, b₂. This fact allows an SS algorithm to be used in        performing detector signal deconvolution within zone O_(i),        resulting in augmented spatial resolution in this zone. This        will now be explained in more detail.

When scanning the surface S of the specimen, one collects the detectoroutput for each scan position, thus forming an “image” D. In thereconstruction scheme discussed here, one is acquiring images using ascanning beam oriented at respective ‘azimuthal’ (yaw) and ‘tilt’(pitch, declination) angles (A, T) with respect to a reference frame(XYZ) whose Z-axis is normal to the surface S (XY plane) of the specimen(azimuthal angle A not depicted in FIG. 2). These images are herelabeled D_(A,T). For simplicity, the following will limit itself to asituation wherein the azimuthal angles are separated by 180° (oppositedirections). Two images are acquired (either serially or concurrently)at respective tilt angles T₁ and T₂ (a special case is T₁=T and T₂=−T),and these two images are respectively labeled D_(T) ₁ and D_(T) ₂ . ForD_(T) ₁ , the collection of detected signals at the visited scanlocations is obtained from the collection of volumes covered by the PSFF₁. In the case of D_(T) ₂ , the imaging process uses the PSF F₂.Obtaining a sharper image, coming from a set of less extended volumes inthe specimen, can be achieved by ‘shifting’ one of the tilt images (forexample D_(T) ₁ ) by a certain number of “pixels” with respect to theother image. The amount of such shift will determine the subsurfacelocation of a volume of intersection of the two PSFs F₁ and F₂, whichintersection volume is labeled O_(i) in FIG. 2A. When there is noX-shift, said intersection volume O_(i) lies immediately underneath thesurface S. Extracting the signal coming exclusively from the collectionof intersection volumes O_(i) (for all scan locations) can be done usingstatistical source separation techniques such as Independent ComponentAnalysis (ICA), for example.

For further explanation, one can define:

-   -   F₁′=F₁\O_(i) as the volume covered by the PSF F₁ excluding        intersection region O_(i); and    -   F₂′=F₂\O_(i) as the volume covered by the PSF F₂ excluding        intersection region O_(i).        The three volumes corresponding to F₁′, F₂′, and O_(i) are        non-overlapping, and one can define three “virtual images” D_(F)        ₁ ′, D_(F) ₂ ′, and D_(O) _(i) corresponding to these virtual        PSFs (volumes). Such virtual images will have lower statistical        correlation than the original images.        In the case of linear imaging models, one obtains:

D _(T) ₁ =D _(F) ₁ ′+D _(O) _(i)   (1)

D _(T) ₂ =D _(F) ₂ ′+D _(O) _(i)   (2)

This model applies in the case where the beam parameters (such ascurrent/particle flux or acceleration voltage) are identical for the twoscans (probing sessions). If one of the beams has a different“intensity” (i.e. greater or lesser importance/influence/“gravity”), arelative weight a given to the intersection region will change from avalue of 1.0, resulting in the more general expressions:

D _(T) ₁ =D _(F) ₁ ′+αD _(O) _(i)   (3)

D _(T) ₂ =D _(F) ₂ ′+D _(O) _(i)   (4)

or, in matrix form:

D=WD′  (5)

with:

$\begin{matrix}{{D = \begin{pmatrix}D_{T_{1}} \\D_{T_{2}}\end{pmatrix}},{D^{\prime} = \begin{pmatrix}D_{F_{1}}^{\prime} \\D_{F_{2}}^{\prime} \\D_{O_{i}}\end{pmatrix}},{{{and}\mspace{14mu} W} = \begin{bmatrix}1 & 0 & \alpha \\0 & 1 & 1\end{bmatrix}}} & (6)\end{matrix}$

The problem to be solved here consists of recovering the virtual imagesD_(F) ₁ ′, D_(F) ₂ ′, and D_(O) _(i) from the ‘observed’ ones D_(T) ₁and D_(T) ₂ . The main emphasis, as explained earlier, will be on D_(O)_(i) , which should correspond to a sharper image. Solving the generallyill-posed decomposition/factorization problem set forth in equation (5)can (for example) be done using ICA techniques with regularizationmethods. The general problem is formulated as follows:

Find the pair (Ŵ, {circumflex over (D)}′) that satisfies:

(Ŵ,{circumflex over (D)}′)=argmin_(W,D′) J(D∥WD′)  (7)

where the criterion J(.∥.) is a statistical similarity measure betweenmodel and observations. Typical choices for J(.∥.) are the least squaresmeasure for signals with Gaussian noise, or the Kullback-Leiblerdivergence for Poisson noise, for instance. Other divergences can beused as well: see, for example, the above-referenced U.S. Pat. No.8,581,189 (item (ii) above), col. 5, line 33-col. 6, line 33.

For better convergence, and to restrict the space of solutions, aregularization term can be added to (7), yielding:

(Ŵ,{circumflex over (D)}′)=argmin_(W,D′) {J(D∥WD′)+λR(W,D′,θ)}  (8)

The regularization term R(W, D′, θ) represents prior-knowledgeconstraints, and may depend on the decomposition variables (W, D′) aswell as on other parameters θ that emerge from simulations andmeasurements. For example, one could use:

-   -   Parameters for an analytical model of a scanning beam's        interaction with a specific class of materials (such as        plastic-embedded heavy-particle-stained specimens, for example);    -   Parameters constraining the geometry and contrast of the        reconstructed structures: for example, synaptic vesicles are, on        average, spherical and have a diameter of approximately 39.5 nm,        etc.

One can see that, in problems (7) and (8), optimizing with respect to Wboils down to optimizing with respect to a. Ultimately, as mentionedearlier, the most important component to recover is D_(O) _(i) , whichis the image corresponding to the smaller “intersection volume”.

An Alternating Least Squares (ALS) Algorithm

What follows is an example of solving for (7) in the case of a leastsquares measure. In this case, one solves for:

(Ŵ,{circumflex over (D)}′)=argmin_(W≧0,D′≧0) ∥D−WD′∥ ²  (9)

which is typically regularized using non-negativity conditions, asindicated in (9) by the stipulations W≧0,D′≧0. This problem can beapproached by alternating two minimization steps, with respect to W andD′. In the first step, one computes a derivative with respect to W andsets it to zero:

$\begin{matrix}{\frac{\partial{{D - {WD}^{\prime}}}^{2}}{\partial W} = 0} & (10)\end{matrix}$

From which one obtains:

D′=(W ^(T) W)⁻¹ W ^(T) D  (11)

Differentiating the least squares criterion with respect to D′ andsetting to zero leads to:

W=DD′ ^(T)(D′D′ ^(T))⁻¹  (12)

In the ALS algorithm, the two steps (11) and (12) are alternated until asuitable convergence criterion is achieved. The aforementionednon-negativity constraints are imposed, for example, by setting negativevalues to zero, or by employing a so-called active set technique, asdescribed (for example) in references [1] and [2] above.ICA with Kullback-Leibler Divergences (EMML)

In this case, one minimizes the following so-called GeneralizedKullback-Leibler (KL) divergence:

$\begin{matrix}{{{KL}\left( D||{WD}^{\prime} \right)} = {{\Sigma_{x}{D(x)}{\log \left( \frac{D(x)}{{WD}^{\prime}(x)} \right)}} - {{\Sigma_{x}\left( {D - {WD}^{\prime}} \right)}(x)}}} & (13)\end{matrix}$

where x is a variable spanning the image coordinates space. In such aformulation, the recovery of the different image components and of theweights matrix is typically achieved using the Expectation MaximizationMaximum Likelihood (EMML) algorithm (see reference [3] above, forexample). This algorithm is built on the following two iterations:

$\begin{matrix}{W_{ij} = {W_{ij}\left( \frac{{\Sigma_{k}\left( {D_{ik}/\left\lbrack {WD}^{\prime} \right\rbrack_{ik}} \right)}D_{jk}^{\prime}}{\Sigma_{k}D_{jk}^{\prime}} \right)}} & (14) \\{D_{jk} = {D_{jk}^{\prime}\left( \frac{\Sigma_{i}{W_{ij}\left( {D_{ik}/\left\lbrack {WD}^{\prime} \right\rbrack_{ik}} \right)}}{\Sigma_{i}W_{ij}} \right)}} & (15)\end{matrix}$

where the indices i, j, and k are used to refer to elements of thedifferent matrices, and where left hand quantities are values beingcomputed at iteration t+1, and right hand quantities are values obtainedat iteration t.

FIG. 2B has already been discussed in the general Description above, andrequires no further elucidation here.

EMBODIMENT 2

FIG. 1A renders a cross-sectional view of a specimen that is beingimaged according to another embodiment of the current invention. Asalready alluded to above, this Figure schematically depicts thefollowing:

-   -   A first incoming radiation beam B₁ that impinges on (original)        specimen surface S, and produces a PSF F₁ that extends down into        the specimen from S₁; point p_(i) is located at a Z-distance L+z        into this PSF F₁.    -   A second incoming radiation beam B₂ that impinges on (newly        exposed) specimen surface on S₂ and produces a PSF F₂ that        extends down into the specimen from S₂; point p_(i) is located        at a Z-distance z into this PSF F₂. This new surface S₂ was        created by utilizing a physical slicing procedure (such as ion        milling, microtome cutting, etching, etc.) to remove a layer of        thickness L from old surface S₁, thereby exposing a fresh        surface S₂.    -   The PSFs F₁ and F₂ demonstrate a partial overlap zone O_(i).        Beams B₁ and B₂ have a common propagation axis b₁₂, which        extends parallel to the Z-direction. The overlap zone O_(i)        containing p_(i) will be subjected to a different Z-region of        each PSF. This fact allows (for example) ICA to be used in        performing detector signal deconvolution within zone O_(i),        resulting in augmented spatial resolution in this zone. This        will now be explained in more detail.

In the case of reconstructions from multiple imaging sessions atdifferent surfaces, one can follow the same reasoning as in Embodiment 1above and define three PSFs: F₁′=F₁\O_(i), F₂′=F₂\O_(i), and O_(i). FromFIG. 1A one can see that, in this case, the “virtual images” D_(F) ₁ ′,D_(F) ₂ ′, and D_(O) _(i) correspond to volumes stacked from the top tothe bottom. Each of these “virtual” volumes will have less thicknessthan the two original ones, and hence should correspond to sharperimages with fewer volume effects. If one defines the images obtained atsurfaces S₁ and S₂ as D_(S) ₁ and D_(S) ₂ , respectively, one can derivethe relationship between the observed and “virtual” images as follows:

D _(S) ₁ =D _(F) ₁ ′+D _(O) _(i)   (16)

D _(S) ₂ =D _(F) ₂ ′+D _(O) _(i)   (17)

If one accounts for a change in imaging conditions that introducesdifferent “intensity” scaling α between the two sessions, one obtains:

D _(S) ₁ =D _(F) ₁ ′+αD _(O) _(i)   (18)

D _(S) ₂ =D _(F) ₂ ′+D _(O) _(i)   (19)

which leads to the matrix representation:

D _(S) =WD′(20)

with

$D_{S} = \begin{pmatrix}D_{S_{1}} \\D_{S_{2}}\end{pmatrix}$

and W and D′ defined as in (6).Using the same ICA techniques as described above, one can recover thethree images corresponding to the three layers from top to bottom.

FIGS. 1B and 1C have already been discussed in the general Descriptionabove, and require no further elucidation here.

EMBODIMENT 3

It will be well within the ambit of the skilled artisan to extend thereconstruction techniques described above to more than two scans(probing sessions). For the multi-tilt case of Embodiment 1, forexample, the different PSFs should in that case intersect in the samecommon (“pivot”) region. The linearity of the imaging process willalways result in decompositions represented by systems of equationssimilar to (5) and (20). Such systems can be solved using identicalcomputational methods.

EMBODIMENT 4

FIG. 3A renders a cross-sectional view of a specimen that is beingimaged according to yet another embodiment of the current invention. Asalready alluded to above, this Figure schematically depicts thefollowing:

-   -   A first the incoming radiation beam B₁, which comprises a first        species of particle. When this beam B₁ impinges on S, it        produces a PSF F₁ that extends to a relatively deep level into        the specimen, but with relatively little lateral spread. This        PSF F₁ intersects point p_(i).    -   A second incoming radiation beam B₂, which comprises a second        species of particle. When this beam B₂ impinges on S, it        produces a PSF F₂ that extends to a relatively shallow level        into the specimen, but with greater lateral spread than in the        case of beam B₁. This PSF F₂ also intersects point p_(i).    -   Since PSFs F₂ and F₁ differ in size and/or shape, point p_(i)        will experience different regions/portions/aspects of each. This        fact allows (for example) ICA to be used in performing detector        signal deconvolution within zone O_(i), resulting in augmented        spatial resolution in this zone.        The reconstruction mathematics in this case are broadly similar        to the general framework set forth in Embodiments 1 and 2 above.        This particular situation is one in which the relative weighting        factor α introduced above in equations (3) and (18) can play an        important role, because of the (typical) dissimilarity between        PSFs F₁ and F₂.

FIG. 3B has already been discussed in the general Description above, andrequires no further elucidation here.

EMBODIMENT 5

FIG. 4 is a highly schematic depiction of an embodiment of ascanning-type microscope according to the present invention; morespecifically, it shows an embodiment of a charged-particle microscope400, which, in this case, is a SEM. The microscope 400 comprises aparticle-optical column 402, which produces a beam 404 of input chargedparticles (in this case, an electron beam). The particle-optical column402 is mounted on a vacuum chamber 406, which comprises a specimenholder/stage 408 for holding a specimen 410. The vacuum chamber 406 isevacuated using vacuum pumps (not depicted). With the aid of voltagesource 422, the specimen holder 408, or at least the specimen 410, may,if desired, be biased (floated) to an electrical potential with respectto ground.

The particle-optical column 402 comprises an electron source 412 (suchas a Schottky gun), lenses 414, 416 to focus the electron beam 404 ontothe specimen 410, and a deflection unit 418 (to perform beamsteering/scanning of the beam 404). The apparatus 400 further comprisesa controller/computer processing apparatus 424 for controlling interalia the deflection unit 418, lenses 414, 416 and detectors 100, 420,and displaying information gathered from the detectors 100, 420 on adisplay unit 426. In the current context, items 414, 416 and 418 may beregarded as comprising the illuminator referred to above.

The detectors 420, 100 are chosen from a variety of possible detectortypes that can be used to examine different types of output radiationflux emanating from the specimen 410 in response to irradiation by theinput beam 404. In the apparatus depicted here, the following detectorchoices have been made:

-   -   Detector 100 is a segmented electron detector. Such a detector        can, for example, be used to investigate the angular dependence        of a flux of output (secondary or backscattered) electrons        emerging from the specimen 410.    -   Detector 420 is a boron-doped solid state detector that is used        to detect (at least a portion of) a flux of output electrons        emanating from the specimen 410.        As here rendered, both detectors 100 and 420 are used to examine        electrons; however, this is purely a design/implementation        choice and, if desired, one could also elect to detect other        types of output radiation flux emanating from the specimen 410        (e.g. X-rays, cathodoluminescence) in addition, or as an        alternative, to electrons.

By scanning the input beam 404 over the specimen 410, outputradiation—comprising, for example, a flux of X-rays,infrared/visible/ultraviolet light, secondary electrons and orbackscattered (BS) electrons—emanates from the specimen 410. As suchoutput radiation is position-sensitive (due to said scanning motion),the information obtained from the detectors 100, 420, will also beposition-dependent. This fact allows the output of detector 420 to beused to produce (for example) a BS electron image of (part of) thespecimen 410, which image is basically a map of an output of detector420 as a function of scan-path position on the specimen 410.

The signals from the detectors 100, 420 are processed by the controller424, and displayed on display unit 426. Such processing may includeoperations such as combining, integrating, subtracting, false colouring,edge enhancing, and other processing known to the skilled artisan. Inaddition, automated recognition processes (e.g. as used for particleanalysis) may be included in such processing.

It should be noted that many refinements and alternatives of such aset-up will be known to the skilled artisan, including, but not limitedto:

-   -   The use of dual beams—for example an electron beam 404 for        imaging and an ion beam for machining (or, in some cases,        imaging) the specimen 410;    -   The use of a controlled environment at the specimen 410—for        example, maintaining a pressure of several mbar (as used in a        so-called Environmental SEM) or by admitting gases, such as        etching or precursor gases, etc.

In the specific context of the current invention, the controller424—and/or a dedicated separate processing unit (not shown)—can beinvoked to perform the following actions in respect of a given pointp_(i) within the specimen 410:

-   -   In a first probing session, employing a first beam configuration        B₁ to irradiate the point p_(i) with an associated first Point        Spread Function F₁, whereby said beam configuration is different        to said measurement parameter;    -   In at least a second probing session, employing a second beam        configuration B₂ to irradiate the point p_(i) with an associated        second Point Spread Function F₂, whereby:        -   F₂ overlaps partially with F₁ in a common overlap zone O_(i)            in which point p_(i) is located;        -   F₁ and F₂ have respective non-overlapping zones F₁′ and F₂′            outside of O_(i),    -   Using a Source Separation algorithm in said computer processing        apparatus to perform image reconstruction in said overlap zone        O_(i) considered separately from said non-overlapping zones F₁′        and F₂′.

Although the scanning-type microscope shown in FIG. 4 is a SEM, itcould, in the context of the current invention, just as validly be aSTEM, FIB-SEM or confocal microscope, for example.

EMBODIMENT 6

FIG. 5 depicts an experimental set-up and imagery pertaining to anexemplary embodiment of the current invention, as will now be explainedin more detail.

FIG. 5A shows a beam geometry used to irradiate a surface S of aspecimen. This figure depicts five different incident beamconfigurations, as follows [whereby notation “(A, T)” indicates“(azimuth angle, tilt angle)”, as respectively measured clockwise from X(viewed parallel to Z) and w.r.t. XY plane]:

-   -   Beam B₁: normal incidence.    -   Beam B₂: angled incidence at (A, T)=(180°, 60°).    -   Beam B₃: angled incidence at (A, T)=(270°, 60°).    -   Beam B₄: angled incidence at (A, T)=(0°, 60°).    -   Beam B₅: angled incidence at (A, T)=(90°, 60°).

FIGS. 5B and 5C show SEM imagery of a specimen of mouse brain tissue,imaged with the following tool settings and in accordance with theset-up shown in FIG. 5A:

-   -   Acceleration voltage: 2 kV.    -   Beam current: ca. 400 pA.    -   Depicted Field of View: 1.5 μm×1.1 μm.    -   Resolution: 4 nm/pixel.    -   Detected flux species: Backscattered electrons.

FIG. 5B shows the image obtained by considering beam B₁ only.

FIG. 5C shows the image obtained by combining all five beams andperforming ICA-based image reconstruction in accordance with the presentinvention (see Embodiment 1 above, for example).

It is immediately clear that the detail/resolution in FIG. 5C (currentinvention) is very superior to that of FIG. 5B (prior art).

EMBODIMENT 7

FIG. 6 depicts imagery pertaining to another exemplary embodiment of thecurrent invention. The specimen in this case comprises rabbit lungtissue, imaged with the following tool settings:

-   -   Acceleration voltage: 2 kV.    -   Beam current: ca. 400 pA.    -   Depicted Field of View: 1.05 μm×0.98 μm.    -   Resolution: 4 nm/pixel.    -   Detected flux species: Backscattered electrons.

FIG. 6A shows a conventional FIB-SEM image of the specimen, using aprior-art imaging technique.

FIG. 6B shows the image obtained by performing ICA-based imagereconstruction in accordance with the present invention, using an“irradiate-slice-irradiate” approach such as that set forth inEmbodiment 2/FIG. 1A above, for example (the FIB functionality of theemployed tool being used to perform ion milling on the specimen betweenirradiation runs, removing a layer of thickness L=ca. 4 nm).

It is again clear that that the image in FIG. 6B (current invention) iscrisper than that of FIG. 6A (prior art).

1. A method of accumulating an image of a specimen using a scanning-typemicroscope, comprising the following steps: Directing a beam ofradiation from a source through an illuminator so as to irradiate asurface S of the specimen; Using a detector to detect a flux ofradiation emanating from the specimen in response to said irradiation;Causing said beam to follow a scan path relative to said surface; Foreach of a set of sample points in said scan path, recording an outputD_(n) of the detector as a function of a value P_(n) of a selectedmeasurement parameter P, thus compiling a measurement set M={(D_(n),P_(n))}, where n is a member of an integer sequence; Using computerprocessing apparatus to automatically deconvolve the measurement set Mand spatially resolve it so as to produce reconstructed imagery of thespecimen, characterized in that, considered at a given point p_(i)within the specimen, the method comprises the following steps: In afirst probing session, employing a first beam configuration B₁ toirradiate the point p_(i) with an associated first Point Spread FunctionF₁, whereby said beam configuration is different to said measurementparameter; In at least a second probing session, employing a second beamconfiguration B₂ to irradiate the point p_(i) with an associated secondPoint Spread Function F₂, whereby: F₂ overlaps partially with F₁ in acommon overlap zone O_(i) in which point p_(i) is located; F₁ and F₂have respective non-overlapping zones F₁′ and F₂′ outside of O_(i),Using a Source Separation algorithm in said computer processingapparatus to perform image reconstruction in said overlap zone O_(i)considered separately from said non-overlapping zones F₁′ and F₂′.
 2. Amethod according to claim 1, wherein Said surface is defined to extendparallel to an XY-plane of a Cartesian coordinate system XYZ; Said beamconfiguration is chosen to be a Z-position of a point of entry of thebeam into the specimen; Between said first and second probing sessions,a physical slicing procedure is used to remove a layer of material ofthickness L from an initial surface S₁, thereby exposing a new surfaceS₂. Point Spread Functions F₂ and F₁ are displaced relative to oneanother in said Z-direction by an amount L.
 3. A method according toclaim 1 or 2, wherein: Said beam configuration is chosen to be an angleof the beam relative to the surface S; Between said first and secondprobing sessions, said angle of the beam is adjusted. Point SpreadFunctions F₂ and F₁ are angled relative to one another.
 4. A methodaccording to any of claims 1-3, wherein: Said beam configuration isselected to be a species of particle in said beam; Point SpreadFunctions F₂ and F₁ are mutually different as regards at least one ofsize and shape.
 5. A method according to any of claims 1-4, wherein saidmeasurement parameter is selected from the group comprising: An averagelanding energy of particles in said beam; An average current of chargedparticles in said beam; An emission angle of particles in said flux; Anemission energy of particles in said flux, and combinations hereof.
 6. Amethod according to any preceding claim, wherein said flux of radiationemanating from the specimen comprises at least one species selected fromthe group comprising backscatter electrons, secondary electrons, X-rays,infrared light, visible light, ultraviolet light, and combinationshereof.
 7. A method according to any preceding claim, wherein saiddeconvolution and spatial resolution of the measurement set M areperformed by minimizing a statistical divergence between a detectionmodel and the measurement set M, assumed subject to at least one ofPoisson noise and Gaussian noise, whilst applying constraints to saidmodel.
 8. A method according to any of any preceding claim, wherein saidSource Separation algorithm is selected from the group comprisingIndependent Component Analysis, Principal Component Analysis,Non-Negative Matrix Factorization, and combinations and hybrids hereof.9. A scanning-type microscope comprising: A specimen holder, for holdinga specimen; A source, for producing a beam of radiation; An illuminator,for directing said beam so as to irradiate said specimen; A detector,for detecting a flux of radiation emanating from the specimen inresponse to said irradiation; Scanning means, for causing said beam totraverse a scan path relative to a surface of the specimen; Acontroller, for: recording an output D_(n) of the detector as a functionof a value P_(n) of a selected measurement parameter P, for each of aset of sample points in said scan path, thus compiling a measurement setM={(D_(n), P_(n))}, where n is a member of an integer sequence;automatically deconvolving the measurement set M and spatially resolvingit so as to produce reconstructed imagery of the specimen, characterizedin that, in respect of a given point p_(i) within the specimen, saidcontroller can be invoked to execute the following steps: In a firstprobing session, employ a first beam configuration B₁ to irradiate thepoint p_(i) with an associated first Point Spread Function F₁, wherebysaid beam configuration is different to said measurement parameter; Inat least a second probing session, employ a second beam configuration B₂to irradiate the point p_(i) with an associated second Point SpreadFunction F₂, whereby: F₂ overlaps partially with F₁ in a common overlapzone O_(i) in which point p_(i) is located; F₁ and F₂ have respectivenon-overlapping zones F₁′ and F₂′ outside of O_(i), Use a SourceSeparation algorithm to perform image reconstruction in said overlapzone O_(i) considered separately from said non-overlapping zones F₁′ andF₂′.